Many studies have conventionally been made to reduce fuel consumption by reducing rolling resistance (expressed also as “by improving rolling resistance performance”) of tires. For example, treads of tires are formed in a two-layer structure (an inner layer and a surface layer), and a base tread of the inner layer is made of a rubber composition having excellent low-exothermic property. Further reduction in fuel consumption has been more and more strongly required in recent years, however, and a rubber composition for a tread among various components in a tire, which occupies a large portion of a tire, is required to have more excellent low-exothermic property.
Known as a method for imparting sufficient low-exothermic property to a rubber composition for a tread is a method in which an amount of a reinforcing filler in the rubber composition is reduced. In such a case, however, hardness of the rubber composition decreases and thereby the tires obtained are softened, which disadvantageously causes decrease in handling performance (handling stability) and wet-skid resistance of automobiles and causes decrease in abrasion resistance of the tires.
Conventionally, a rubber composition, which contains butadiene rubber for improving flex crack resistance and carbon black for improving weather resistance and a reinforcing property in addition to a natural rubber having excellent tear strength, has been used as a rubber composition for a sidewall of a tire. As mentioned above, however, the further reduction in fuel consumption has been more and more strongly required, so that not only a tread but also a sidewall is required to have improved rolling resistance performance.
For reducing rolling resistance of a sidewall, a method in which a content of a reinforcing filler is reduced has been known as well as for a tread. Unfortunately, however, strength of a rubber composition decreases and thereby the tire obtained becomes susceptible to damage.
A natural rubber is used in various tire components because it has high mechanical strength and excellent abrasion resistance. In the case of the natural rubber, however, reversion owing to over-vulcanization tends to occur and, disadvantageously, fuel consumption may easily increase. For inhibiting reversion and improving heat resistance of vulcanizable rubber compositions used in rubber products such as tires, a technique in which a blending amount of a vulcanization accelerator is increased relative to an amount of sulfur as a vulcanizing agent, a technique in which a thiuram-type vulcanization accelerator is blended as a vulcanization accelerator, and other techniques have been conventionally known.
It also has been known that reversion of a rubber composition can be inhibited by blending, in the rubber composition, a crosslinking agent capable of forming a long chain crosslinking structure represented by —(CH2)6—S—. With respect to the crosslinking agent, PERKALINK 900 and Duralink HTS (each produced by Flexsys), and Vulcuren KA9188 (produced by Bayer AG) are known. Such a technique can inhibit reversion; however, fuel consumption increases and mechanical strength decreases, which disadvantageously causes deterioration of balance of performances.
Patent Document 1 discloses a rubber composition for a tire containing anhydrous silica and hydrous silica together for the purpose of improving wet-skid resistance without decreasing abrasion resistance and increasing fuel consumption. However, there is still a room for improvement in low-exothermic property.
Patent Document 1: JP 2003-192842 A